Several practical applications require a user device to detect and track spread spectrim signals. For example, a wireless location-determination unit (herein "GPS Unit") which uses Global Positioning System (GPS) satellites to determine its approximate location must detect and track multiple spread spectrum signals from the GPS satellites.
Detection of spread spectrum signals, such as those transmitted by the constellation of GPS satellites, is typically performed using a search procedure which detects the signals in both frequency and time. When searching for the appropriate time offset, a GPS Unit may sequentially "step" or "sweep" through a range of time offsets from a reference waveform. Alternatively, a parallel process using a Fast Fourier Transform (FFT) or Surface Acoustic Wave (SAW) filter could be used.
The appropriate time offset is identified as the offset when the GPS Unit 5 detects a peak in a correlation function between an incoming GPS signal and the reference waveform. Correlation of the incoming signal and the reference waveform is typically defined by averaging the product of the incoming signal with a locally-synthesized reference waveform in the GPS Unit's receiver.
When wideband spread spectrum signals are used for communication or for precision range measurement, the detection and acquisition of the desired signal requires that the search process be restricted to very small time steps. Consequently, extensive search resources are required for acquisition when the receiver clock has any significant timing uncertainty (i.e., when a wide time range is searched).
As stated previously, the GPS Unit also performs a search in the frequency domain, thus requiring additional search resources. Similarly to the time offset search, the frequency search typically sweeps or steps through a range of frequencies to detect a peak in a frequency correlation function.
The frequency search process is made more difficult when the incoming signal is very weak. An incoming signal may be weak for several reasons. For example, the signal may have been transmitted at a low power, the distance between the transmitter and the GPS Unit may be great, or the GPS Unit may be in an environment where signal reception is attenuated (e.g., inside a building). For very weak signals, the signal dwell time must be sufficient to provide detectability, thus placing restrictions on the frequency step that must be searched.
Because of the restrictions on the frequency step and the timing step, the efficiency of two dimensional (i.e., frequency and time) searches is a primary issue for efficient detection and acquisition of complex spread spectrum waveforms.
What is needed are a method and apparatus to more efficiently detect and track spread spectrum signals, particularly weak spread spectrum signals. Particularly needed are a method and apparatus to rapidly determine location using GPS signals in environments where the incoming GPS signals are attenuated.